The present invention deals with a method and a means to prevent or reduce cavitation erosion in propeller ducts, particularly in so-called "Kort nozzles".
On ships equipped with propeller ducts, cavitation and/or erosion is often created on the inside surface of the duct adjacent or beyond the blade tips. This erosion is due to the so-called "gap cavities" formed at the blade extremities or tips and between blade tips and duct surface.
For smaller vessels this type of cavitation erosion is seldom a serious problem and can generally be avoided by correct hydrodynamic design of propeller and duct or by using stainless steel in the duct surface close to the blade tips. Since 1972 ducted propellers have also been applied to the very large tankers of more than 200 000 TDW. During 1973 and 1974 it has become clear that for such ships cavitation erosion in the duct is a more difficult problem to solve than in the case of smaller vessels. Various investigations are going on in 1974 to solve the problem by improved hydrodynamic design of the propeller and duct. An attempt has also been tried to solve the problem by using various materials or coatings in the duct surface close to the propeller blade tips. Such methods may well be a final solution for ships to be designed in the future. However, ships that are already in service and are suffering from this problem will not benefit from the above mentioned results.